1. Field of the Invention
The present invention relates to apparatus and methods for oil and gas wells to enhance the production of subterranean wells, either open hole, cased hole, or cemented in place and more particularly to improved multizone stimulation systems.
2. Description of Related Art
Wells are drilled to a depth in order to intersect a series of formations or zones in order to produce hydrocarbons from beneath the earth. Some wells are drilled horizontally through a formation and it is desired to section the wellbore in order to achieve a better stimulation along the length of the horizontal wellbore. The drilled wells are cased and cemented to a planned depth or a portion of the well is left open hole.
Producing formations intersect with the well bore in order to create a flow path to the surface. Stimulation processes, such as fracing or acidizing are used to increase the flow of hydrocarbons through the formations. The formations may have reduced permeability due to mud and drilling damage or other formation characteristics. In order to increase the flow of hydrocarbons through the formations, it is desirable to treat the formations to increase flow area and permeability. This is done most effectively by setting either open-hole packers or cased-hole packers at intervals along the length of the wellbore or cementing in the horizontal liner. When using packers the packers isolate sections of the formations so that each section can be better treated for productivity. Between the packers is a frac port and in some cases a sliding sleeve or a casing that communicates with the formation. In order to direct a treatment fluid through a frac port and into the formation, a seat or valve may be placed close to a sliding sleeve or below a frac port. A ball may be dropped to land on the seat in order to direct fluid through the frac port and into the formation.
One method, furnished by PackersPlus, places a series of ball seats below the frac ports covered by sliding sleeves with each seat size accepting a different ball size. Smaller diameter seats are at the bottom of the completion and the seat size increases for each zone as you go up the well. For each seat size there is a ball size so the smallest ball is dropped first to clear all the larger seats until it reaches the appropriate seat. In cases where many zones are being treated, maybe as many as 20 zones or more, the seat diameters have to be very close. The balls that are dropped have less surface area to land on as the number of zones increase. With less seat surface to land on, the amount of pressure you can put on the ball, especially at elevated temperature, becomes less and less. This means you can't get adequate pressure to frac the zone because the ball is so weak, so the ball blows through the seat. Furthermore, the small ball seats reduce the I.D. of the production flow path which creates other problems. The small I.D. prevents re-entry of other downhole devices, i.e., plugs, running and pulling tools, shifting tools for sliding sleeves, perforating gun size (smaller guns, less penetration), and of course production rates. In order to remove the seats, a milling run is needed to mill out all the seats and any balls that remain in the well.
The size of the ball seats and related balls limits the number of zones that can be treated in a single trip.
It would be advantageous to have a system that had no ball seats that restrict the I.D. of the tubing and to eliminate the need to spend the time and expense of milling out the ball seats, not to mention the debris created by the milling operation. Also, it would be advantages to eliminate the restricted flow paths due to the small I.D. of the ball seats that could potential restrict production.
Another method of completion is called “Plug and Perf”. In these completions the liner may be cemented in throughout the length of the horizontal section. Typically, composite plugs are run into the well on electric line and pumped out the horizontal section toward the toe until the composite plug is below the section of the zone to be fraced. Once at the desired location, a setting tool is actuated and the composite plug sets inside of the liner. Perforating guns are sometimes run in the same electric line trip where once the composite plug is set, the guns and setting tool release away from the composite plug and are moved up to a location where the liner is perforated with the guns. Once perforated, the spent perforating gun and setting tool are returned to the surface. Frac fluid is then pumped into the well in order to frac the zone. After treatment, the next composite plug with setting tool and perforating guns is run to the next upper zone section and the process described above is repeated and obviously this becomes very time consuming. This process can be repeated many times and in some cases up to 40 times. Once all zones have been fraced, a coiled tubing unit runs coiled tubing into the well with a motor and mill attached and all of the composite plugs are milled out. The composite plug mill debris is flowed back to the surface and the well is put on production.
It would be advantageous, and cost effective, to have a system and method where no wireline trucks were required to perform electric line runs to run and set composite plugs, perforate, and return tools to the surface. Furthermore, it would be advantageous and cost effective, to eliminate the need to call a coiled tubing unit to location to mill out the composite plugs.
The “Plug and Perf” method is sometimes desired over the sliding sleeve method because last minute changes can be made on zone spacing since the composite plugs can be set at any location along the length of the well. The present invention offers a solution to making position changes of the plug in the liner at the last minute by use of selective key profiles located at each liner coupling. Casing liner comes in length increments ranging from 30 to 40 feet and typical stage zone lengths vary maybe from 300 feet to 500 feet. So, for example, a 300 foot zone may have about 9 selective profiles to choose from when anchoring or positioning a cup plug. Therefore, a plug key profile can be selected at the surface to match a liner coupling profile where the plug is desired to land and anchor in place. The plug key profile can be designed to pass certain liner coupling profiles until the plug finds the correct profile. The plug key profile is also designed to easily disengage from the from the liner coupling profile so that flow from the well will return the plug back to the surface thus eliminating the need to mill out the plug.
The “Plug and Perf” method can also use a conventional wireline conveyed perforating gun attached to the top of the cup plug. In this case the cup plug serves a dual purpose, i.e., first, conveys the tools to a location, and second, provides a seal to frac against. It would be advantageous to use the cup plug as a power means to pump the perforating guns out the horizontal wellbore and land the cup plug in a profile to positively locate the guns along the horizontal section. The perforating guns could be detached from the cup plug by different means, i.e., apply pump pressure to the cup and jar up on the guns to release from the plug or incorporate an electrical triggered release device between the guns and the cup plug. Once the perforating gun is released from the cup plug, the gun is positioned at selected locations above the plug. A single shot gun can be used or a select fire can be used to generate a series of perforation clusters within a zone.
The invention is not to be limited to wireline, or electric line, conveyed guns attached to the cup plug since a pressure actuated type gun can be attached to the top of the cup plug providing safety issues can be resolved in cases where the guns do not fire and have to return to the surface for disarmament. This would be advantageous since a wireline trip would be eliminated.